FLINT, Mich. (AP) — A criminal investigation of Flint's lead-contaminated water turned to former key officials at City Hall on Tuesday as Michigan's attorney general announced charges against four people accused of keeping residents on a contaminated system that caused the crisis.

Darnell Earley and Gerald Ambrose separately were state-appointed emergency managers in Flint in 2014-15 when the city was using the Flint River as a source of drinking water. Ambrose also served earlier as a financial adviser to the troubled town.

They were charged with four crimes, including conspiracy and misconduct in office. Howard Croft, Flint's former public works director, and Daugherty Johnson, the former utilities director, were charged with conspiracy and false pretenses.

Attorney General Bill Schuette said Earley and Ambrose committed Flint to $85 million in bonds to join a new regional water pipeline to Lake Huron while at the same time using a city water plant that was not equipped to properly treat the river water before it went to roughly 100,000 residents.

They claimed that debt-burdened Flint needed to sell bonds to clean up a lagoon, Schuette said, but the money went as the city's share to Karegnondi Water Authority to build the pipeline, which still is under construction.

"This case is a classic bait-and-switch. ... The lime sludge lagoon was not an emergency," said special prosecutor Todd Flood.

During a news conference, there was no allegation by Schuette that Earley and Ambrose personally gained from the bond deal or by keeping the Flint River as the source of water for Flint while the pipeline was being constructed.

Flint's water system became contaminated with lead because water from the river wasn't treated for corrosion for 18 months, from April 2014 to October 2015. The water ate away at a protective coating inside old pipes and fixtures, releasing lead.

Schuette said the investigation has revealed a "fixation on finances and balance sheets" in Flint during that period.

"This fixation has cost lives," he said, noting that 12 people died from Legionnaires' disease, which has been linked by experts to the river water. "This fixation came at the expense of protecting the health and safety of Flint. It's all about numbers over people, money over health."

Earley, Ambrose and Croft could not immediately be reached for comment. They didn't appear in court Tuesday.

Johnson is "going to plead not guilty and we're going to stand by that," attorney Edwar Zeineh said.

Lansing Mayor Virg Bernero released a statement, defending Ambrose as a "man of the highest character who would never knowingly endanger the public health."

The latest charges bring to 13 the number of people who have been charged in the investigation of Flint water and the Legionnaires' outbreak. The other nine are eight current or former state employees and a water plant employee.

Perhaps the most significant catch so far: Corrine Miller, Michigan's former director of disease control, pleaded no contest to willful neglect of duty in September. She said she was aware of dozens of cases of Legionnaires' in the Flint area around the same time the city changed its water source, but she didn't report it to the general public.

"The investigation has continued to go up and go out. ... We are not at the end," said former FBI agent Andy Arena, the lead investigator.

Meanwhile, tests show Flint's water quality is improving, although residents are urged to drink tap water only if it's first run through a filter.